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# Hedos

Member Since 03 Oct 2002
Offline Last Active Feb 21 2012 11:00 PM

### #4879793I have a problem with theory of relativity

Posted by on 02 November 2011 - 12:12 PM

This explains why the clock on the destination planet reads 20 years from the ship-based observer, even though he knows the clock must have only been ticking for 15 years since the ship left Earth. It's because from the ship's reference frame, the two clocks were not started at the same time and the clock on the destination planet has been started 5 years before the ship's clock was started. This part can be calculated using a Lorentz transformation to compare the space-time coordinates of an event in different inertial reference frames.

Confusing stuff... But you are saying that the guy left on the planet will read 15 years on the same clock?
What happens when you land your spaceship?
Will the decelleration make the clocks "synchronize", otherwise guy A on planet and B on the ship will have a big problem deciding on a time to meet eachother...
If you never land, but have some kind of way to communicate, would guy A and B start to argue about what time the clock is showing?
Or would communication be impossible?

The observer on the planet will read 15 years on the ship's clock, when the ship arrives, yes. This is the same time that the ship-based observer read on the ship's clock. The ship-based observer reads 20 years on the planet's clock and the planet-based observer reads 20 years on the planet's clock. The two observers are in complete agreement.

Whether you land the spaceship or not doesn't matter! Nothing would change at all. There is no synchronization involved and there is no contradiction either.

Time has indeed been shorter for the observer travelling on a spaceship, there is asymmetry in this situation (because the spaceship undergoes changes of velocity but the planet does not).

Keep in mind that when the spaceship has just been launched from Earth, then the reference frame of the Earth and the reference frame of the spaceship disagree about when the clock on the destination planet was started relative to when the ship-based clock was started. But that's because the ship-based clock and the destination planet's clock are physically separated. When the ship arrives at its destination, then because the two clocks are at the same location, it doesn't matter at all which frame of reference you choose, they will all be in agreement about the two clocks.

### #4879427I have a problem with theory of relativity

Posted by on 01 November 2011 - 04:35 PM

Okay, I think I understand your confusion. For simplicity, let's assume that the ship is going at speed 0.661c relative to the two planets, hence the time and space dilation factor is gamma = 3/4.

From the reference frame of the two planets, the ship is moving, hence the ship's clock is ticking slower. With a speed of 0.661c the clock on the ship will show approximately 15 years when it arrives at the destination planet due to time dilation. As others have pointed out, an observer on the planet will read the ship's clock as indicating 15 years too and an observer on the ship will also read the clock on the ship as indicating 15 years.

Now, from the reference frame of the ship, however, it is the destination planet that is moving. Hence, the clock on the planet is ticking slower. So at a speed of 0.661c, it is correct to say that the clock on the planet will only have ticked for 15 years throughout the journey and in the reference frame of the spaceship.

I think your mistake is to assume that the clock on the planet will read 15 years from the observer on the spaceship. That is wrong, the clock will read 20 years from the observer on the spaceship, as others have pointed out.

The problem lies with the notion of simultaneity of events. In relativity, there is no such thing as absolute simultaneity of events. So you cannot say without ambiguity that the two clocks were started at the same time. You need to specify in which frame of reference the clocks are being synchronized. So if the clocks were started at the same time in the reference frame of the planets, then in the reference frame of a spaceship that has just been launched from Earth, the two clocks were not started at the same time, in fact the clock on the destination planet was started earlier.

This explains why the clock on the destination planet reads 20 years from the ship-based observer, even though he knows the clock must have only been ticking for 15 years since the ship left Earth. It's because from the ship's reference frame, the two clocks were not started at the same time and the clock on the destination planet has been started 5 years before the ship's clock was started. This part can be calculated using a Lorentz transformation to compare the space-time coordinates of an event in different inertial reference frames.

### #4819650Mini-Contest: ASCII Fishtank

Posted by on 05 June 2011 - 12:27 AM

I'm going to enter this contest. I have some free time this month, plus this looks like fun

I strongly suggest anybody that's willing to participate to have a look at Benryves' Windows Console tutorial to learn how to setup a 80x25 or 80x50 console window without a scrollbar, with no flickering and with user input.

So far I have bubbles and one type of fish that only moves in a straight line

### #4819117Debate me about the bible

Posted by on 03 June 2011 - 10:13 AM

The belief of athiesm is nothing new. [...]

I'd just like to make a small note here, because this is something that I've heard often lately and that I find irritating. The sentence "the belief of atheism" is a misrepresentation of what atheism is. Atheism is the lack or rejection of a belief in some form of a god (this is *not* agnosticism). The sentence "the belief of atheism" has implications, it is meant to suggest that atheism is an equivalent philosophical stance as is the belief in other religions, because it also requires a leap of faith. That is not the case, because the burden of proof is on religions. They make extraordinary claims, so they require extraordinary proofs. Atheism does not make any extraordinary claim.

This statement above is essentially the same as the statement that an atheist would need to "prove that atheism is true". Would you require me to prove to you that Santa Claus does not exist? Would you require me to give you proof that the Tooth Fairy or that the Flying Spaghetti Monster don't exist? Of course not. If you choose to believe in any such fairy tale, you are the one expected to provide very strong evidence.

This is probably not something you did on purpose, but I noticed some people constantly using this rhetoric and I thought this should be clarified.

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